The present invention is generally directed to a process for increasing the wet tensile strength of highly porous paper, such as porous plug wraps used in the construction of smoking articles. More particularly, the present invention is directed to a process for increasing the wet strength of porous plug wraps by adding a ketene dimer to a pulp fiber suspension during formation of the paper in combination with adding polyvinyl alcohol to the surface of the paper after the paper has formed.
Smoking articles such as cigarettes are conventionally made by wrapping a column of tobacco in white wrapping paper. At one end, the smoking article usually includes a filter through which the article is smoked. Filters are attached to smoking articles using a tipping paper which is glued to the white wrapping paper. Although there are some exceptions, conventional filters are typically formed from either compressed strips of paper or from cellulose acetate tows. Between the filter material and the tipping paper exists a porous plug wrap that holds the filter together prior to attachment to the wrapped column of tobacco.
The paper components used to produce smoking articles not only provide structure but also contribute to or control many physical properties and characteristics of the cigarette. For instance, the paper can be used to control the rate at which the cigarette burns, the number of puffs per cigarette, and the total tar delivery per puff. Many of the above properties are controlled by producing paper with a particular permeability.
For instance, porous plug wraps are lightweight papers that have a very high porosity and permeability. In particular, porous plug wraps are produced with high permeabilities so that they do not interfere with the burn characteristics of the smoking article as is more appropriately controlled by the cigarette wrapping paper and the tipping paper.
Unfortunately, since plug wraps need to have a very high porosity, plug wraps have relatively low strength which presents problems during formation of the paper and during formation of the smoking article.
For instance, during the formation of smoking articles, adhesive glue is typically applied to the porous plug wrap paper to serve as anchor lines for the cylindrical filter tow. The adhesive can wet the paper where applied and can thus reduce the strength of the paper. It is important that the paper is strong enough to withstand the forces that are being exerted on it, especially from the compacted filter tow.
The plug wrap material is folded tightly to encompass the rod of tow material. Specifically, the resulting plug wrap rod is passed through a garniture which brings the rod to the desired diameter. After the filter tow is wrapped by the plug wrap paper, the filter is then subjected to heat in order to dry the paper and the adhesive. Subsequently, the filter is cooled. During this step in the manufacturing procedure, another problem experienced during the formation of the filter is that during cooling, the filter tow and the plug wrap paper can pick up condensation. The condensation weakens the paper and can cause the plug wrap to expand. This expansion coupled with the loss of strength of the porous plug wrap paper can cause failure in production due either to circumference variation or a complete breach in the plug wrap.
In view of the above potential problems and difficulties that are experienced during the production of cigarette filters, in the past those skilled in the art have attempted to either increase the strength of porous plug wraps or make the porous plug wraps more moisture resistant. For instance, sizing agents have been applied to plug wraps in the past in order to increase the amount of water the porous paper can withstand or xe2x80x9chold outxe2x80x9d during exposure to moisture. This method does increase the wet tensile strength of the plug wrap, but it does not increase it to a level sufficient enough to completely eliminate failures during production. Also in the past, strength enhancing agents have been applied to the porous paper. Examples of such agents include some cross-linking agents and wet strength resins. Unfortunately, these additional agents have recently come under environmental scrutiny in various countries.
Thus, a need still remains for a plug wrap paper for smoking articles that has improved wet tensile strength characteristics. Also, a need exists for a method of producing such plug wrap paper. A need also exists for a process that not only improves the xe2x80x9chold outxe2x80x9d of plug wraps during exposure to moisture but also increases the wet tensile strength of the porous plug wraps. A need further exists for a method of increasing the wet tensile strength of plug wraps without the addition of cross-linking agents and certain wet strength resins.
The present invention recognizes and addresses the foregoing disadvantages, and others of prior art constructions and methods.
In general, the present invention is directed to a process for increasing the wet tensile strength of porous plug wrap paper used in the construction of smoking articles. The wet tensile strength of the plug wrap paper is increased by the addition of a ketene dimer composition in conjunction with a polyvinyl alcohol size. Through this process, the wet tensile strength of the plug wrap paper can be increased without the addition of cross-linking agents. As will be described in more detail hereinafter, the method of the present invention can be used to automatically increase the wet tensile strength as the porous plug wrap paper is being made.
Accordingly, it is an object of the present invention to provide an improved method of increasing the wet tensile strength of porous plug wraps.
Another object of the present invention is to provide a method for increasing the wet tensile strength of plug wraps while eliminating the addition of cross-linking agents and/or certain wet strength resins.
It is another object of the present invention to provide a method for automatically increasing the wet tensile strength of a porous plug wrap paper as the paper is being made.
These and other objects of the present invention are achieved by providing a process for increasing the wet tensile strength of porous plug wrap paper for a smoking article. The process includes the steps of incorporating a ketene dimer composition into the paper followed by applying a polymeric binder size, such as a polyvinyl alcohol size. The sequence of the additions is important to the enhancement of the wet tensile strength of the paper.
According to the present invention, the wet tensile strength of porous plug wrap paper can be increased above conventional plug wraps without the use of a cross-linking agent and/or wet strength agents that have recently fallen under environmental scrutiny. In particular, plug wrap papers made in accordance with the present invention have demonstrated a wet tensile strength retention greater than 20% and more particularly between about 20% to 35%. The basis weight of the porous plug wrap can be between about 10 g/m2 to about 40 g/m2 and more particularly between about 15 g/m2 to about 25 g/m2. For most applications, a plug wrap can be formed having a permeability, for instance, no less than 600 Coresta units and more preferably over 3000 Coresta units.
The ketene dimer composition and polymeric binder size added to the plug wrap paper can be an alkyl ketene dimer and polyvinyl alcohol respectively.
In one embodiment, the alkyl ketene dimer can be added in an amount from about 0.00001% to about 1% by weight based upon the weight of fiber contained in the paper. The polyvinyl alcohol can be a non-borated polyvinyl alcohol added in an amount from about 0.5% to about 15% by weight.
These and other objects of the present invention are also achieved by providing a process for increasing the wet tensile strength of porous plug wrap paper as the paper is being made. The system includes a paper forming device to form a continuous sheet of paper from a fiber suspension. The alkyl ketene dimer is added at the wet end of the papermaking process, specifically to the papermaking pulp slurry such as prior to the head box or at the head box. The polyvinyl alcohol is then added to the dry end of the papermaking process as a size press coating after a set of dryers.
Other objects, features and aspects of the present invention are discussed in greater detail below.